3. It would add time

My understanding is that once he signs it, the signed document must be hand delivered back to some clerk of congress, so the law can be entered and become effective. If he signed it in Hawaii and put it on a plane back to DC, it would just add another day to the process, plus its a waste of money.

He is the first POTUS to use it to sign laws. Most since JFK have used it for ceremonial signatures.